1. Field of the Invention
The present invention relates to an electrode structure for silicon carbide (SiC) semiconductors. More particularly, the invention relates to an ohmic electrode structure for SiC semiconductor devices useful as devices operable at high temperatures, devices for use with great electric power, radiation-resistant devices, photoelectric conversion devices, etc.
2. Description of the Prior Art
Many crystal structures are present for SiC, which has forbidden band widths of 2.2 to 3.3 eV depending on the crystal structure. Further SiC is very stable thermally, chemically and mechanically, and can be of p-type and n-type both with high stability, which is rarely the case with wide-gap semiconductors. Accordingly, semiconductor devices comprising an SiC single crystal which is provided with an electrode for electrical connection to external circuits will find wide use in various fields of electronic techniques as devices operable at high temperatures, devices for use with great electric power, radiation-resistant devices, photoelectric conversion devices, etc.
Conventionally, the electrode to be formed on SiC semiconductors is prepared from nickel (Ni) for n-type SiC or from eutectic crystal of aluminum (Al) and silicon (Si) for p-type SiC, by depositing the material on SiC by vacuum evaporation and converting the deposited film to an alloy in contact with SiC at a high temperature of about 1100.degree. C. ("Breakdown Field in Vapor-Grown Silicon Carbide P-N Junctions," Journal of Applied Physics, Vol. 48, No. 11, pp. 4831-4833 (1977)).
When an electrode is to be formed on SiC semiconductors in this way, the electrode film deposited requires heat treatment at a high temperature of about 1100.degree. C. to form an alloy, whereas the electrode metal undergoes agglomeration during the conversion, making it difficult to afford a uniform ohmic electrode. Furthermore, the agglomeration of the electrode metal stresses the SiC single crystal in contact therewith to result in lower crystallinity due to the distortion of the crystal, increased dislocation, etc. This impairs the electrical contact between the SiC single crystal and the electrode and further entails troubles such as separation of the electrode from the SiC semiconductor when the device has a large area, thus posing the problem that the semiconductor device fails to retain the desired reliability.
On the other hand, as far as we are aware, no report has been made on an electrode formed by depositing Ti, Al or the like on an SiC semiconductor by vacuum evaporation. Further, it has been speculated that in view of the properties to be exhibited by Ti, Al or the like on Si, such element will provide a schottky contact to act as a schottky electrode (SEMICONDUCTORS HANDBOOK, revised edition, pp. 315-317, printed by OHM Corporation, Japan (1984)).
Under the foregoing situation, we have conducted extensive research and found that when Ti, Al or the like is deposited on an SiC semiconductor by vacuum evaporation, a compact film can be formed with excellent adhesion and denseness. Moreover, we have unexpectedly found that the film functions as an ohmic electrode.